A plane in heavy fog at Shuangliu Airport in China's south-western city of Chengdu on Sunday. More than 10,000 passengers were stranded at the airport in the capital of China's Sichuan province due to the dense fog that prompted the authorities to issue a red alert - the highest in a four-tier system - for the first time since the airport opened in the 1950s. More than 130 flights were delayed and two were cancelled, according to the South China Morning Post. The airport, ranked the fourth busiest on China's mainland last year with an average 805 flights a day, resumed operations on Sunday afternoon.